1. Field of the Invention
The present invention relates to a shift control system of an automatic transmission for a vehicle.
2. Description of the Prior Art
Japanese Patent Application Laid-open No. 7-301321 discloses an example of a shift control system of an automatic transmission for a vehicle. As disclosed, this conventional system is provided with an automatic shift mode and a manual shift mode. When a manual shift mode is selected, an upshift operation and a downshift operation are performed by the driver""s shift operation, and thus gear ratios are continuously controlled by the driver""s manual operation. On the other hand, when a shift lever is placed in a neutral position between an upshift and a downshift, a gear ratio is fixed. This makes it possible to adjust driving force and engine braking force continuously and to perform a speed change operation in accordance with the driver""s intention.
However, a problem with the aforementioned conventional shift control system of an automatic transmission is that shift operation must be always performed in order to obtain a desired engine speed and driving force while vehicle speed changes variously.
The present invention has been made in view of the aforementioned problem, and its object is to provide a shift control system of an automatic transmission capable of performing a speed change operation in accordance with intention of a driver and also changing speed without performing a troublesome shift operation.
In order to achieve the above object, according to an aspect of the invention, there is provided a shift control system for an automatic transmission which has a gear ratio control means for controlling gear ratio of the automatic transmission by determining a target gear ratio from a drive point determined on the basis of at least a throttle opening and a vehicle speed, comprising a lower limit setting device provided in the gear ratio control means and setting a lower limit for inhibiting a gear ratio from changing to a gear ratio of an upshift side; and a lower limit variable control means in which a lower limit of the gear ratio can be changed to a downshift side or to an upshift side by an operation of a driver.
In the aspect of the invention, by setting a lower limit of a gear ratio and changing this lower limit by the driver""s operation, engine braking force can be optionally obtained and a running in a xe2x80x9cDxe2x80x9d range can be performed at a lower ratio side than the set lower limit. In the automatic transmission having a conventional manual shift function, shift operation must be always performed in order to obtain a desired engine speed and driving force while vehicle speeds changes variously. On the other hand, according to the invention, even in case a lower limit of a selected gear ratio is set, a normal running in the xe2x80x9cDxe2x80x9d range can be performed within a range of a lower ratio side than the selected gear ratio, and therefore the driver""s shift operation load can be reduced.
In the preferred embodiment, the shift control system may change a control state between first state in which an automatic shift can be performed regardless of the lower limit and second state in which an automatic shift can be performed within a gear ratio range which is not below the lower limit.
As a result, it is possible to perform gear ratio controls that reflect the driver""s intention more sufficiently.
Preferably, the lower limit setting device of the gear ratio control means sets a gear ratio at the time of changing from the first state in which an automatic shift can be performed regardless of the lower limit into the second state in which an automatic shift can be performed within a gear ratio range which is not below the lower limit, as the lower limit.
As a result, it is possible to avoid occurrence of shift shocks when the automatic gear ratio control is shifted to the lower limit variable control.
Further preferably, the lower limit variable control means includes a lowest side limiting ratio for restricting a change range of a downshift side of the lower limit, a highest side limiting ratio for restricting a change range of an upshift side of the lower limit and a speed upper limit of a transmission input shaft within a change range of the lower limit, and when a speed of the transmission input shaft reaches the speed upper limit due to a gear ratio selected within the change range, the lower limit variable control means inhibits the lower limit from shifting from the selected gear ratio to a gear ratio of a downshift side, and when a speed of the transmission input shaft exceeds the speed upper limit, the lower limit variable control means shifts the lower limit to a gear ratio of an upshift side that does not cause the speed of said transmission input shaft to exceed said speed upper limit.
As a result, an excessive engine brake can be suppressed, and also a so-called over revolution, that is, a state where the engine rotates excessively, can be inhibited.
Further preferably, the shift control system is equipped with a speed changeable range indicating means for indicating an allowable shift range to a driver when the lower limit variable control means controls a lower limit variably.
As a result, the driver can recognize a position of the lower limit and the allowable shift range.
Further preferably, the shift control system is equipped with a gear ratio selecting means capable of selecting gear ratios by the driver""s shift operation, and changeable a lower limit for the lower limit variable control means by the shift operation thereof; and a switching means for switching between a shift control by the gear ratio selecting means according to the shift operation and a shift control by the lower limit variable control means according to the shift operation.
Thereby, since the conventional manual shift device as it is becomes able to be used as a changing device for a lower limit variable control, both operations of a conventional manual range and a lower limit variable control can be performed with one shift lever.
Further preferably, the shift control system is equipped with a selection condition indicating means for indicating whether the lower limit variable control is selected.
As a result, the driver can recognize that a lower limit variable control is selected even during running in the xe2x80x9cDxe2x80x9d range.
Further preferably, the lower limit variable control means inhibits a lower limit variable control at the time of activation of an anti lock brake system (ABS) for preventing wheels from being locked or slipping due to the drivers hard braking.
As a result, it is possible to prevent the wheels from being locked or slipping due to an excessive engine braking force when the anti-lock brake system is activated.
Still further preferably, the automatic transmission is a continuously variable transmission for selecting gear ratios continuously; and the lower limit variable control means changes a lower limit continuously in case where a lower limit change operation is continued for a certain period of time by the driver at the time of change of a lower limit of a gear ratio.
As a result, the driver can change the lower limit continuously while feeling a desired engine braking force. Since shift operation is limited to the adjustment of engine braking force and others, moreover, this shift operation is not troublesome to the driver.
The above and further objects and features of the invention will be more fully apparent from the following description when the same is considered with reference to the accompanying drawings.